# Geometry :

Subject: [ggl] Re: Problems with the difference between two polygons
From: Angus Johnson (awj1958)
Date: 2011-08-07 14:56:44

Hi Enrico.

On 7/08/2011 8:39 AM, Enrico Leoni wrote:
> >It's interesting to see that Clipper returns a smaller numbers of
> vertices
> >in these tests.

> However, in the first test other differences emerge. In the attached
> example (svg file) the black poligon is an intersection and these are
> the vertices returned:
> Clipper
> 51600,0
> 25800,-44686
> 26450,-45812
> 52900,0
>
> Gtl
> 52900,0
> 51600,0
> 25800,-44686
> 25799,-44686
> 26449,-45812
> 26450,-45812
> 52900,0
>
> Ggl
> 52900,0
> 26450,-45812
> 26449.711324944485,-45812
> 25799.615099925981,-44686
> 25800,-44686
> 51600,0
> 52900,0
>

I've rearranged the vertices above (see below) since Ggl's are
counter-clockwise to the other two, and also removed duplicate vertices
and I get the following ...

Gtl:
51600,0
25800,-44686
25799,-44686
26449,-45812
26450,-45812
52900,0

Ggl:
51600,0
25800,-44686
25799.615099925981,-44686
26449.711324944485,-45812
26450,-45812
52900,0

Clipper:
51600,0
25800,-44686
26450,-45812
52900,0

You can see that if the vertices returned by Ggl are then rounded
they'll produce duplicate vertices. Once they're removed, the result is
exactly that which is returned by Clipper. So it seems that Clipper is
returning the correct and simplest solution.

I'm not sure of the significance of your SVG image. It looks very
different to the image I get from the coordinates above. Is this showing
some sort of error with Clipper??

> >Also, depending
> >on the degree of precision you're using in these tests, I suspect that
> >Clipper could be optimised by setting the UseFullCoordinateRange
> property to
> >false.
>
> I wasn't aware of this setting and I am going to use it in the next
> update.

When this is enabled it allows users to enter coordinates using the full
Int64 range (using an internal Int128 class to do math on 64bit integers
safely), but this adds roughly a 20% time penalty.

Thanks for all your efforts in doing these tests. The results are very
interesting. Would you consider adding one more test that benchmarked
self-intersecting polygons too?

Regards
Angus

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